add dynamic bodies inside destructible level

[filbs111]

eg boulders that fall once the destructible earth surrounding them is destroyed. hopefully doable by: subtract path from level. add dynamic body in place. conceivably problematic since the level collision is via edge shapes.

[filbs111]

TODO rock shapes, setup code to define shape once, and use for both clipping and dynamic body.

[filbs111]

done. problem: path array appended to level, which is fine if entirely over solid part of level, but if hole if outside of existing solid area, will add to level. not sure if should use clip instead of append, or if can fix simply by using ClipperLib.PolyFillType